Genotype-soil interactions regulating nutrient use efficiency in winter wheat

Background A large proportion of the C in root exudates is easily degradable and promotes enhanced microbial growth and activity in the rhizosphere. This enhanced activity can lead to improved nutrient availability through: a) increased flux of nutrients by mass flow from the bulk soil, b) 'priming' of recalcitrant organic compounds in the soil releasing bound nutrients, c) release of organic anions and phosphatases that enhance the availability of P, d) development of associations with beneficial microorganisms (e.g. free-living N fixers, AM fungi). In this project we will test the hypothesis that there are genotypic differences among wheat varieties in their ability to modify conditions in the rhizosphere environment and affect nutrient availabilities within this zone. Aims The specific objectives of the project are: 1. To determine the genotypic variation in wheat rhizosphere processes that mediate the availability of plant nutrients. 2. To determine the genotypic variation in the activity of wheat rhizosphere-based beneficial organisms. 3. To determine which phenotypic traits are linked to improved nutrient availability in the rhizosphere of wheat. 4. To link the phenotypic traits to genetic markers for nutrient use efficiency in wheat. Brief experimental plan Objective 1: Rhizosphere processes will be monitored in 2010 and 2011 in the NUE-CROPS trials at Newcastle University, and 2011 and 2012 in the Sustainable Arable LINK project (SA Root Ideotypes, awaiting final approval from BBSRC) trials at KWS UK. Measurements will include: a) soil microbial activity, b) microbial biomass size, c) enzyme activities (e.g. carbohydrases, acid phosphatase, leucine-aminopeptidase) using the microfluorometric assay. The student will rhizosphere sample at both experimental sites, with lab activities at Newcastle University. Objective 2: Previous studies have demonstrated that free-living N fixing bacteria are sensitive to management practices and that the effects of management can be detected using state-of-the-art molecular techniques (RNA-qPCR and RNA-DGGE). The structure and activity of the free-living N fixing bacterial population in the rhizosphere will be studied using these techniques. Studies of other key functional groups in the soil (e.g. ammonia oxidizers, AM fungi) will also be conducted if the enzyme studies under objective 1 indicate differential activity of these groups. The soil molecular studies will be conducted at Newcastle University. Objective 3: The field trials (NUE-CROPS and SA Root Ideotypes) will be used to estimate the nutrient use efficiency of the different wheat genotypes. A range of phenotypic assessments (e.g. root system size, whole plant N partitioning) will be linked to nutrient use efficiency. In addition, the quantity and effects of root exudation will be studied to further elucidate the mechanisms driving genotypic differences in rhizosphere soil processes. Microcosm studies will be conducted on selected varieties (those identified as exhibiting differences in rhizosphere biology under objectives 1 and 2) using stable C isotope labelling techniques. This will allow tracing of root exudates into the rhizosphere and microbial populations. Field based phenotypic assessments will be conducted at both experimental sites; microcosm studies will be conducted at Newcastle University. Objective 4: The rhizosphere parameters assessed under objectives 1, 2 and 3 will be included as phenotypic measurements that can be linked to ongoing transcriptional and proteomic studies of wheat roots and shoots in the NUE-CROPS project. The most robust rhizosphere parameter(s) showing variation between the 64 genotypes will be used with the large array of genetic markers available from the NUE-CROPS project (~800 DArT / SSR) to identify candidate locations for QTL using association mapping methods.